1. Field of the Invention
The present invention relates to a feed rod for growing a magnetic single crystal and to a magnetic single crystal.
2. Description of the Related Art
With respect to high-frequency devices, such as isolators, circulators, and magnetostatic wave devices, since there are demands for miniaturization and higher performance, the need for magnetic single crystals is increasing. Magnetic single crystals are also used for magneto-optical devices, such as optical isolators, magneto-optical sensors, and optical modulators using optical high frequency coupling.
In order to produce bulk single crystals, various methods have been proposed and put into practical use, and examples thereof are the Czochralski method (hereinafter referred to as the xe2x80x9cCZ methodxe2x80x9d) and the floating-zone method (hereinafter referred to as the xe2x80x9cFZ methodxe2x80x9d).
A light-focusing and heating method in which heating is performed by focused light is a representative of the FZ method. In the light-focusing and heating method, a raw material polycrystal (feed rod) is held in a furnace, and a predetermined zone thereof is melted by heating at a temperature that is higher than the melting point to form a molten zone. By moving the molten zone, the molten zone is cooled and solidified, thus growing a single crystal. This method is advantageous in that high purity of the crystal is maintained because the crystal is not brought into contact with a crucible and that the growth rate of the crystal is high because of a steep temperature gradient. As the FZ method in which the crystal is not brought into contact with a crucible, for example, a laser-heating pedestal growth method (hereinafter referred to as a xe2x80x9cLHPG methodxe2x80x9d) using a combination of heating by focused light and laser heating is disclosed in Japanese Unexamined Patent Application Publication No. 6-48883.
Examples of materials used for high-frequency devices, such as isolators, include an yttrium-iron-garnet (Y3Fe5O12; hereinafter referred to as YIG) single crystal. The YIG is an incongruent melting compound, and it is not possible to directly produce a single crystal having the same composition simply by cooling and solidifying a melt having a stoichiometric composition. That is, when the YIG melt having the stoichiometric composition is solidified, an orthoferrite (YFeO3) precipitates as the initial phase, and at approximately 1,585xc2x0 C., the orthoferrite reacts with the liquid phase to form YIG. Therefore, in the conventional FZ method, since the orthoferrite deposits on the joint between the feed rod and the seed crystal, it is not possible to control the growth orientation of the single crystal.
As a method capable of controlling the growth orientation of the single crystal, a traveling solvent floating zone method (hereinafter referred to as a xe2x80x9cTSFZ methodxe2x80x9d), which is another variation of the floating zone method, is known. In this method, a solvent whose composition and weight are precisely controlled is placed on a seed crystal, and after the solvent and the seed crystal are fully fused with each other by heating, a raw material polycrystal (feed rod) is joined thereto to grow a single crystal. In this method, it is possible to grow a bulk single crystal of an incongruent melting compound from the melt while controlling the growth orientation. Therefore, recently, the TSFZ method has been widely used to produce YIG single crystals.
However, when an attempt is made to produce a single crystal in the shape of a fiber having a diameter of 2 mm or less by the TSFZ method, it is physically difficult to place a solvent on the seed crystal, and moreover, since the amount of solvent is very small, it is difficult to control the weight thereof. For the reasons described above, if the amount of the solvent becomes unsuitable, the growing conditions for the single crystal become unstable, and the growing single crystal is broken due to the remaining orthoferrite layer or the excessive amount of solvent.
In order to produce a fiber-like single crystal, a self-solvent floating zone method (hereinafter referred to as a xe2x80x9cSSFZ methodxe2x80x9d) is disclosed in Japanese Unexamined Patent Application Publication No. 10-251088, which overcomes the problems described above. In this method, an end of a raw material polycrystal (feed rod), i.e., the end opposite to the end to be joined to the seed crystal, is heated and melted to cause an incongruent melting reaction, and the generated liquid phase is moved to the seed crystal side to be joined to the seed crystal. In the SSFZ method, a fiber-like single crystal is grown while controlling the growth orientation by placing the orthoferrite (YFeO3) outside the reaction system.
However, when a YIG single crystal is produced by the SSFZ method, if the growth rate is increased in order to improve the productivity, the orientation controllability of the growing single crystal is lost, the ferromagnetic resonance half-value width (xcex94H) is significantly increased, and the magnetic properties are remarkably degraded. Therefore, the YIG single crystal must be grown at a rate of 10 mm/hour or less, resulting in low productivity and high cost.
In order to overcome the problems described above, preferred embodiments of the present invention provide a feed rod for growing a magnetic single crystal capable of producing a YIG single crystal in which the orientation controllability of the single crystal is maintained, the ferromagnetic resonance half-value width (xcex94H) is not increased, and the magnetic properties are not degraded even if the growth rate is increased, and to provide a magnetic single crystal.
A preferred embodiment of the present invention comprises a feed rod for growing a magnetic single crystal having a composition represented by the formula (Y3-aAa)(Fe5-b-cBb)O12-xcex1, wherein A is at least one element selected from the lanthanoide series, B is at least one element selected from the group consisting of Ga, Al, In, and Sc, c is a value for decreasing the Fe content from the stoichiometric amount, xcex1 is a value for decreasing the oxygen content to satisfy the chemically neutral condition, and the relationships 0xe2x89xa6axe2x89xa70.5; 0xe2x89xa6bxe2x89xa71.0; 0 less than cxe2x89xa70.15 and 0 less than xcex1 are satisfied.
Another preferred embodiment of the present invention comprises a magnetic single crystal having a composition represented by the formula (Y3-aAa)(Fe5-b-cBb)O12-xcex1, wherein A is at least one element selected from the lanthanoide series, B is at least one element selected from the group consisting of Ga, Al, In, and Sc, c is a value for decreasing the Fe content from the stoichiometric amount, xcex1 is a value for decreasing the oxygen content to satisfy the chemically neutral condition, and the relationships 0xe2x89xa6axe2x89xa70.5; 0xe2x89xa6bxe2x89xa71.0; 0 less than cxe2x89xa70.15 and 0 less than xcex1 are satisfied.
By decreasing the Fe content in the feed rod for growing the YIG single crystal from the stoichiometric amount by the value c and by decreasing the oxygen content by the value xcex1 to satisfy the chemically neutral condition, even if the growth rate of the YIG single crystal is increased, it is possible to obtain a YIG single crystal in which the orientation controllability is maintained and which has satisfactory magnetic properties with a small ferromagnetic resonance half-value width (xcex94H). Consequently, the productivity is improved and it is possible to supply stable and inexpensive magnetic single crystals.